Chapter 13, Trigonometric Ratios and Functions - 13.4 Evaluate Inverse Trigonometric Functions - 13.4 Exercises - Quiz for Lessons 13.3-13.4 - Page 880: 10

$-90^{\circ}$ or, $-\dfrac{\pi}{2}$

Here, we have $\cos^{-1} (-1)$ This means that $\theta=\sin^{-1} (-1)$ This gives : $\theta=-90^{\circ}$ The range of the function $arccos$ is from $[-90^{\circ}, 90^{\circ}]$. Hence, we have $\theta=-90^{\circ}$ or, $-\dfrac{\pi}{2}$